Highly Stable Three-Dimensional Nickel–Cobalt Hydroxide Hierarchical Heterostructures Hybridized with Carbon Nanotubes for High-Performance Energy Storage Devices

A three-dimensional (3D) composite consisting of nickel–cobalt (Ni–Co) dual hydroxide nanoneedles (NCDHNs) grown on a carbon nanotube (CNT) material, denoted as CNTs@NCDHNs, was designed using a facile one-step hydrothermal method. This composite was further fabricated into electrodes, which exhibited high rate capability and long cycle life. Comparative analysis of the electrochemical performance between 3D CNTs@NCDHNs electrodes and Ni–Co hydroxide electrodes revealed that the high rate capability and long cycle life of the CNTs@NCDHNs are due to a synergistic effect. The CNTs@NCDHNs exhibited a high specific capacitance of 1823 F g–1 at a current density of 1 A g–1, and more than 77.6% of the capacitance was retained at a charge–discharge rate of 20 A g–1. To evaluate the functional behavior of the CNTs@NCDHNs, quasi-solid-state cells using CNTs@NCDHNs as the positive electrode and rGO–Fe2O3 as the negative electrode were assembled and tested. These devices presented ultrafast charge–discharge rates of up to 20 A g–1 with high rate capabilities and excellent long-term cyclic stability. The corresponding quasi-solid-state device presented a high energy density of up to 54.6 Wh kg–1 at a power density of 1.13 kW kg–1 and an energy density of 35.8 Wh kg–1 at 12.4 kW kg–1 when a voltage in the range 0–1.6 V was applied. Moreover, the device exhibited optimal flexibility, stability, and safety under different extreme conditions.